The present invention relates to a deproteinizing agent for removing a protein in a natural rubber, a deproteinized natural rubber latex obtained by using the same, and a method of producing a rubber product using the rubber latex.
Natural rubbers have widely been used in various fields, for example, industrial goods such as automobile tire, belt and adhesive, household goods such as glove, medical appliances such as catheter, lactation appliances, contraceptive device and the like because of features such as large extension, high elasticity and strong film strength.
Among these, glove, medical appliances and lactation appliances are products obtained by dipping a mold for these rubber products in a natural rubber latex and these dipped products are produced by using (a) a so-called direct method of directly dipping a mold in a natural rubber latex, (b) a so-called anode coagulation method of coating a mold with an anode coagulant and dipping the mold in a natural rubber latex, and (c) a so-called heat sensitizing method of dipping a previously heated mold in a natural rubber latex containing a heat sensitizer, thereby gradually depositing a gel on the surface of the mold.
These preparation methods are appropriately selected according to the kind of dipped products. The direct method is employed when producing products such as condom made of a rubber film having a very small thickness, the anode coagulation method is employed when producing a household glove made of a rubber film having a thickness of about 1 mm, and the heat sensitizing method is employed when producing more thick products such as work glove.
By the way, it has recently been reported that, when using medical appliances made of the natural rubber, such as surgical glove and various catheters, immediate (I type) allergy, which shows symptoms such as respiratory distress and anaphylactoid symptom (e.g. vascular edema, urttication, collapse, cyanosis, etc.) within several hours, is caused. It is presumed that such immediate allergy is caused by a protein, as an antigen, in the natural rubber.
Also it has been known that the protein contained in the natural rubber causes variations in quality and vulcanization properties of the natural rubber because the kind and quantity of the protein vary depending on the locality and production season of the natural rubber latex. In addition, the protein can cause deterioration of mechanical characteristics such as creep characteristics and aging resistance and electrical characteristics such as insulating properties.
Accordingly, it has been considered to be important to highly remove a protein from a natural rubber latex and to obtain a so-called deproteinized natural rubber latex wherein the protein is highly removed.
Japanese Published Unexamined Patent (Kokai Tokkyo Koho Hei) No. 6-56902 discloses a method of adding a proteolytic enzyme (protease) and a surfactant to a natural rubber latex, thereby decomposing a protein, and separating a creamy deproteinized natural rubber component by centrifugation.
According to this method, the protein in the natural rubber latex can be removed in a very high level and the nitrogen content (N %) is reduced to 0.1% by weight or less as measured by the Kjeldahl method.
However, a large amount of a surfactant is incorporated into the deproteinized natural rubber latex obtained by the method described above in order to prevent destabilization of the latex due to removal of the protein and to prevent coagulation of rubber particles. Therefore, the dispersion state of rubber particles in the latex is excessively stable.
Therefore, the direct method of directly dipping the mold in the natural rubber latex to form a film causes liquid dripping, thereby making it difficult to form a uniform thin film. Namely, it is difficult to produce a dipped product having a very small film thickness, such as condom, in a uniform film thickness by using the deproteinized natural rubber latex.
The deproteinized natural rubber latex obtained by the method described above has a problem that coagulation of rubber particles can not be effected by a conventional heat sensitizer, thereby making it impossible to form a film according to the formulation of a conventionally used heat sensitizing method.
There is also a problem that a surfactant present in the latex exerts an adverse influence on various physical properties of the rubber product, such as deterioration of the water resistance of the resulting rubber product.
However, the method described in the publication described above has a problem that, since the both of the heat sensitizer and anode coagulant are incorporated into the latex, the latex becomes unstable as compared with a conventional heat sensitizing method using a natural rubber latex, thereby making it impossible to obtain long-term stability and making it hard to control heat-sensitive properties.
The present applicants have previously filed a patent application with respect to such an invention that a dip product having a sufficient film thickness is obtained by using a specific heat sensitizer and a specific anode coagulant in a specific combination and incorporating the specific combination into a deproteinized natural rubber latex wherein rubber particles are excessively stabilized, obtained by the method described above, in a large amount as compared with a conventional formulation (Japanese Published Unexamined Patent (Kokai Tokkyo Koho) No. 2000-17002).
The present applicants have continuously studied to solve such a problem that, since the both of the heat sensitizer and anode coagulant are incorporated into the latex in the method described in the publication described above, the latex becomes unstable as compared with a conventional heat sensitizing method using a natural rubber latex, thereby making it impossible to obtain long-term stability and making it hard to control heat-sensitive properties.
Thus, an object of the present invention is to provide a deproteinizing agent which can realize high deproteinization of a natural rubber latex and provide the latex with sufficient heat-sensitive coagulation properties while maintaining the stability of the deproteinized natural rubber latex for a long term, and to provide a method of preparing a deproteinized natural rubber latex which simultaneously satisfy the stability and heat-sensitive coagulation properties of the latex.
Another object of the present invention is to provide a method capable of producing a uniform dipped product having a very small film thickness from a highly deproteinized natural rubber latex, as a raw material, without causing liquid dripping.
A still another object of the present invention is to provide a method capable of producing a dipped product having a sufficient film thickness from a highly deproteinized natural rubber latex, as a raw material, even in case of the same formulation as that of a conventional heat-sensitive latex using a natural rubber latex, the method capable of easily controlling the heat-sensitive properties.